Most existing light bulbs are incandescent light bulbs or fluorescent light bulbs. An incandescent light bulb typically comprises a base, a glass shell, a thin filament which is normally a thin tungsten filament within the shell, and an inert gas within the shell. When an electric current passes through the tungsten filament and heats it up to an extremely high temperature (2000° C. to 3000° C. depending on the filament type, shape, size, and amount of current passed through), heat radiation occurs and visible light is produced. However, the incandescing process is considered highly inefficient, as over 98% of its energy is emitted as invisible infrared light (or heat) and the luminance cannot further improve. In addition, the typical lifespan of an incandescent bulb is limited to about 1,000 hours.
By comparison, a fluorescent light bulb is filled with gas containing low-pressure mercury vapor and an inert gas such as argon or xenon. The inner surface of the bulb is coated with a fluorescent (and often slightly phosphorescent) coating made of various blends of metallic and rare-earth phosphor salts. When electricity passes through mercury vapour, the mercury vapour produces ultraviolet light. The ultraviolet light is then absorbed by the phosphorus coating inside the bulb, causing it to glow, or to fluoresce. While the heat generated by fluorescent light is much less than its incandescent counterpart, efficiencies are still lost in generating the ultraviolet light and converting this light into visible light. In addition, mercury is considered detrimental to the health of people and animals. Therefore, if the fluorescent bulb breaks, exposure to the substance can be hazardous. Fluorescent bulbs are typically more expensive than incandescent bulbs, but they have life spans of about 10,000 hours.
A light emitting diode light bulb is another type of light bulb. The LED bulb typically has high durability with no need to worry about the filament breaking as occurs with respect to incandescent bulbs or the noted hazards as can occur with respect to fluorescent bulbs. LED light bulbs have a long life span of approximately 50,000 to 100,000 hours. The LED bulb generates little heat and has little parasitic energy loss, thereby reducing the overall electricity used. This, in turn, increases the possibilities of reducing electricity bills. Since the LED light bulb has so many advantages over the incandescent bulb and the fluorescent bulb, it is considered to be a cost-effective yet high quality replacement for incandescent and fluorescent light bulbs.
There are already some LED bulbs in the market. These LED bulbs either contain one LED in the bulb or at least two LEDs horizontally fixed directly on one printed circuit board (PCB) in the bulb. For the bulb containing only one LED, the light is generally not bright enough. The luminance is hard to improve for a single bulb containing a single LED. For bulbs having at least two LEDs horizontally fixed on one PCB, the LEDs are in the same horizontal level and the distances that can be brightened by those LEDs are similar because of their attachment to the PCB. When the bulb shell increases in size, the LEDs will all have a longer distance to the bulb shell. As the distance from the LED to the shell increases, the brightness becomes weaker and dimmer. Light is governed by an increase-square law of physics namely that the intensity/strength of the light from a source is inversely proportional to the square of the distance from the source. Therefore, the use of LED bulbs in the prior art is limited to applications which do not have a high luminance requirement. In order to broaden the use of LED bulbs because of their so many advantages, limited luminance needs improvement.